Apparatus for coating flexible webs



L. O. GOFF July 8, 1947.

Filed. Jan. 13, 1943 Patented July 8, 1947 Lindsay 0. G03, Westbrook, Maine,

S. D. Warren Company, Bos

asslgnor to ton, Mass a corporation of Massachusetts Application January 13, 1943, Serial No. 472,234 7 Claims. (CI. 91-18) This invention relates to the coating of flexible webs such as cloth, paper, Cellophane. and the like.

More particularly, the invention relates to an air brush or air doctor designed to prevent fluttering of the unsupported web when a jet of a suitable gas, hereinafter referred to as air, is directed by it against a coated surface of the web in such a way as to remove excess coating and smooth the residual coating. The invention also relates to apparatus for coating both sides of a web simultaneously involving the new air brush.

The use of an air brush in the coating of a web such as paper, that is, for removing excess and smoothing residual coating, is described in United States patent to S. Le'bel, No. 1,980,923, dated November 13, 1934. United States patent to K. E. Terry, No. 2,139,628, dated December 6, 1938, describes an improved air doctor for this purpose. Both of these patents, however, disclose the smoothing of the coating on the web while it is rigidly supported against the action of the air brush, and the air brushes described therein arenot designed for nor adapted to smooth the coating on an unsupported web. United States patcuts to Merrill No. 2,176,093, dated October 17, 1939, and to L. O. Goff and F. H. Frost No. 2,229,- 921, dated January 28 1941, describe methods and apparatus for simultaneously coating both sides of a web of paper involving the use of air brushes for simultaneously smoothing the coatings. The Merrill patent states that the invention thereof has to do with the use of air jets in somewhat different form and arrangement from that previously used in coating one side of a web and stresses the utility of the associated nozzle 8 and deflector I and the deflectors l2 and l2a for maintaining the web in a fixed central position between the two air brushes 4 and 4a acting on the opposite sides of the web. The Golf et al. patent also employs a special arrangement characterized in that the air jet leaving the nozzle of the air brush passes through a throat deflned by the tensioned web and a conversely curved surface.

The present invention resides in an air brush the leading lipof which presents to the web being treated flrst a recess and then a curved or streamlined surface in the path of the air stream passing between the web and the leading lip of the air brush. The air brush is designed and particularly adapted for smoothing coating simultaneously on both sides of a web but is, of course, useful for smoothing coating on only one side oi an unsupported web and may, if desired, be used for smoothing the coating on one side of a web when the opposite side is rigidly supported as in. the process of Lebel.

For convenience in the following description, thetwo lips of the air brush nozzle will be referred to as the leading lip and the trailing lip, the leading lip being the lip on the side of the nozzle from which the web approaches the nozzle and the trailing lip being the lip on the side of the nozzle on which the traveling web moves away from the nozzle. Also, for convenience in the following description, each air brush nozzle will be considered as being made of a pair of rigid members having surfaces such as are generated by movement of a straight line parallel to itself, said two surfaces facing each other with their lines of generation parallel and defining a throat, and said members terminating in lips which define the slot of the nozzle, said slot being understood to be the space between the lines of closest approach of the pair of surfaces or members.

The invention will be described with reference to a specific embodiment illustrated in the accompanying drawing in which:

Fig. 1 is a diagrammatic side elevation of the essential parts of a paper coating machine,

Fig. 2 is an enlarged diagrammatic section of the lips of two air brush nozzles acting on opposite sides of a web.

Fig. 3 is a diagrammatic section of the portion of an air doctor adjacent the slot showing the leading member formed in a unitary structure, and

Fig. 4 is a diagrammatic section of the portion of an air doctor adjacent the slot showing the leading member provided with an adjustable baflle member.

Referring to Fig. 1 of the drawings, l is the web to be coated, e. g. a paper web, which passes from the unwind reel 2 over the tension rolls 8, which may be driven if desired or necessary, then beneath the surface of liquid coating composition 4 in the pan 5. It then passes betweensqueeze rolls 6 and .I which leave a limited excess of coating composition on both sides of the web, and then between the opposed air brushes}! and 9 which direct jets of air obliquely against their respective sides of the web in a direction opposite to the direction of the web travel. The jets act to remove excess coating and smooth the remalnder. The'excess coating blown back by the jets mainly flows back along the web and down over rolls 6 and 1 into the pan 5. The small portion of the excess coating which is atomized by the jets and carried in suspension in the resulting gas currents is caught by the shield to and returned to the pan I. The web, upon leaving the air brushes 8 and 8, passes over the floater I I which preferably is of the type described in the Merrill Patent 2,176,093 referred to above because of its ability to hold the web in a straight course without vibration and then over the floaters H which may be of any type capable of supporting the web until the coating has dried sumciently that it will not be marred by contact with solid bodies. The web then passes over suction apron i3 which serves in part as means for advancing the web. The web may then be reeled up, further dried as in festoons or otherwise treated as desired, such disposition of the web being beyond the scope of the invention. The in-' vention is, of course, not limited to any particular means for applying coating to the web or for moving the web over the air brushes, the apparatus illustrated in Fig. 1 and described in connection therewith being merely illustrative o a suitable apparatus for effecting these results.

The air brushes 8 and 9 aside from the lips may be of any suitable design and preferably are patterned after the air doctor described in the Terry Patent No. 2,139,628 above referred to. The surface of the trailing lip It, as illustrated, is like the trailing lip of the air doctor described in the Terry patent, that is, the surface of the lip external to the throat and adjacent the slot diverges sharply away from the plane of the web. The trailing lip may, however. have other shapes than the plain wedge shape illustrated as will be described hereinafter.

The leading lip is modified as compared with the leading lip of the Terry air doctor. The leading lip shape illustrated in Fig. 2 is produced by simply securing to the fiat front surface of the leading member I of an air brush of the type described in the Terry patent above referred to, a member or baiile "which presents a curved, specifically a semi-cylindrical, surface ill to the web. The curved surface l8 extends to substantially the same distance from the web as does the edge of members l4 and i5 and forms, with the front flat surface of member i 5, the groove or pocket l9 which serves as an expansion chamber for the air leaving the air brush slot. How this chamber l9 function in cooperation with the rounded surface l8 to eliminate fluttering of the web is not definitely known but a possible explanation is that it serves to reduce the velocity of the air stream and to cushion the web against fluttering while the rounded surface i8 prevents the production of eddy currents in the air stream issuing between it and the web.

The leading lip of the air brush could be made unitary as illustrated in Fig. 3 instead of by adding the strip or bafile I1 to the leading member iii of, an air brush of the type described in the Terry patent. Also the curvature and size of the surfac l8 and the size and shape of the chamber l9 may be modified without departing from my invention. Experience has shown that neither the shape nor the size of either the .surface l8 or the chamber I9 is critical so long as there is a chamber of substantial volume between the edge of the member l5 of the air brush and the curved surface l8 and so long as curved surface IB complies with the well known principles of streamlining and serves to eliminate or substantially to reduce eddy currents in the air leaving the nozzle slot. As shown in Fig. 3 the unitary structure 20 provides the equivalent of the member l5 and baffle ll of the structure illustrated in Fig. 2.

The curved surface it is illustrated as bein semi-cylindrical but it is within the scope oi" my invention to employ other curved shapes such as elliptical or the conventional falling drop contour in'cross section, the only requirement being that the curved surface acting together with the web provides a throat or passage through which the air is discharged without creating eddy currents which cause fluttering of the web. The surface in front of the chamber must therefore be a smooth rounded surface and the curvature mustextend for some distanc beyond the point of closest approach to the web. For precision it may be said that the tangent to the curved surface at its forwardmost point should make an angle of at least about 45 with the tangent to the curved surface at its point of closest approach to the web. The shape of the curved surface in front of its point of closest approach to the web need not, as a matter of fact, be the 'same as that extending from the point of closest approach to the web rearwardly into the chamber I9. This latter surface may be convexly curved toward the web as illustrated or a plane surface or even a concave surface because the desired streamlining effect is essential only forwardly from the point of closest approach of the surface iii to the web. For the same reason, the front surface of member l5 within the chamber or recess l9 may be plane, as illustrated, or of any other suitable shape so long as it and that portion of the surface l8 rearwardly of its point of closest approach to the web form a recess of substantial size, i. e. of the order of size of the recess formed by a plane surface and a cylindrical surface of from about a; to inch diameter which is tangent to the plane surface at a distance about equal to the radius of the cylindrical surface from the edge of the plane surface. The size of the chamber I9 and of the surfac IE will, of course, vary with the size of the slot of the air brush and the volume of air passing through it. The above figures are given for an air brush having a slot about .025 inch wide operated at a pressure of about 1 pound per square inch.

- Due to the fact that the air brush is directed at an oblique angle toward the web, the plane of the jet making an acute angle with the web on the trailing side and an obtuse angle on the leading side, and due to the momentum of the jet, any air which flow between the edge of the trailin lip and the web is inconsequential and the shape of the edge of the trailing lip 15 substantially immaterial. The front or inner face of the trailing lip adjacent the edge nearest the 'web may therefore be either plane or rounded toward the rear face thereof and the surface nearest the web may be either a relatively sharp edge as illustrated or rounded.

The angle of impingement of the jet against the web may be adjusted as described in the Terry patent referred to since the shape of the leading lip does not modify the action of the air brush in this respect. It is to be observed that the lead ing lipextends farther in the direction of the jet than does the trailing lip, that is, the jet strikes the web at an oblique angl and the leading and trailing lips generally are substantially equally spaced from the web.

It will be appreciated that the direction of travel of the web is of little consequenc since the effect of gravity is small as compared with the other forces involved. From the standpoint of disposing of the excess coating removed from the web by the air brushes it would be best for the web to move upwardly but it might be moved downwardly or horizontally or at any angle between vertical and horizontal.

It will be appreciated further that the lips of the two opposed air brushes acting simultaneously on the web provide a rather narrow passageway, for example 0.09 inch, through which the paper must pass without striking the lips of either air brush. It must therefore travel in a closely controlled path, preferably under tension, and must not vibrate enough to strike the lipsof the air brushes or enough to cause an uneven action of the air brushes on the coating, resulting in an uneven coating on the web.

The invention has been illustrated and described with reference to an air brush nozzle of the type of that described in the Terry patent which is designed to provide a non-divergent air jet. It is to be understood, however, that the invention is not limited to this type of nozzle. The air jet, due to the rounding of the leading lip, necessarily will to some extent at least lose some of the non-divergent character of the air jet produced by the air brush described in the Terry patent. It will be appreciated therefore that the invention is applicable to air brush nozzles of other constructions than that disclosed by Terry.

The leading lip may b adjusted so as to be nearer to or farther away from or at the same distance as the distance of the trailing lipfrom the web. Good results have been obtained when the leading and trailing lips are at substantially equal distances from the web.

It may be desirable to mount the baffle ll' by means of bolts 2| and slotted holes as illustrated in Fig. 4 to permit adjustment of the baffle. The rounded surface of the baffle may be positioned either closer to or farther away from the web than the edge of the member I5 or the baffle and the edge may be equally spaced from the web.

Bailles having semi-cylindrical surfaces having radii varying from to inch have been used successfully. Satisfactory results have been obtained, for instance with a bafile having a semicylindrical surface inch in diameterand the baffles of the opposed air brushes spaced 0.06 inch apart.

It will be appreciated that in the actual construction of a coating machine, it is advantageous to mount the air brushes so that they can be adjusted with respect to their distance from each other and also so that they may be tilted to vary the angle of impingement of the air jets on the web, and further so that they may be swung out of the path of the web. These structural features are mechanical details which do not constitute a part of the present invention and have therefore not been illustrated.

I claim:

.1. An air doctor comprising rigid members defining a throat and terminating in lips defining a relatively long and narrow slot for the emission of a jet of gas, one member having a surface adjacent to and extending laterally of said slot and external to said throat and which is recessed with respect to a plane through the edge thereof and perpendicular to the direction of said jet, and a surface which is convexly curved in the direction of the jet and joins said recessed surface.

2. in; air (:1 enter comprising two rigid members defining a throat and terminating in lips defining a relatively long and narrow slot for the emission of a jet of gas, the lip edges of said members being substantially wedge shaped, one of said members having a substantially plane surface external to said throat and adjacent said slot and a baffle member attached to said plane surface, said baille member having a convexly curved surface which meets said plane surface at a line spaced at a substantial distance from the lip edge thereof.

3. An air doctor comprising a leading member and a trailing member defining a throat and terminating in lips defining a slot for the emission of a relatively long and narrow jet of gas, the lip edges of said members being substantially wedge shaped, aid leading member having a sur face which is convexly curved in the direction of the jet and is spaced from the slot by a surface which is recessed with respect to said curved surface and the lip edge of said member.

4. An air doctor comprising rigid members defining a throat terminating in lips defining a slot, the leading member having a surface extending forwardly from the slot, said surface havin a convexly curved portion separated from said slot by a recessed portion.

5. Apparatus as defined in claim 4 in which the leading member of-said air doctor has a relatively sharp edge and a substantially plane surface external to the throat and adjacent said edge and a baille member having a convex substantially semi-cylindrical surface which contacts said plane surface at a distance from said edge substantially equal to the radius of said semi-cylindrical surface.

6. Apparatus as defined in claim 4 in which the leading member has a relatively sharp edge adjacent the slot, a substantially plane surface external to the throat and adjacent said edge and-a baflle member adjustably secured to said surface, said baffle memberv having a convexly curved surface which joins said plane surface at a line of contact spaced from said edge.

7. An air doctor comprising two rigid members having spaced apart opposed surfaces defining a throat, said members terminating in lips defining a relatively long and narrow rectilinear slot. one of said members having a convexly curved surface external to said throat and separated from its lip by a recessed surface, each of said surfaces being capable of being generated by movement of a straight line to successive parallel positions.

LINDSAY O. GOFF.

REFERENCES CITED 1 The following references are of record in the file of this patent;

UNITED STATES PATENTS Number Name Date 2,130,241 MacLaurin Sept. 13, 1938 2,139,628 Terry Dec. 6, 1938 2,236,005 Massey Mar. 25, 1941 2,073,461 Cerwin Mar. 9, 1937 1,899,449 Wallsten Feb. 28, 1933 2,139,628 Terry Dec. 6, 1938 2,176,093 Merrill Oct. 17, 1939 FOREIGN PATENTS Number Country Date 309,866 Great Britain ,1 Jan. 30, 336 

